There is a great demand for polyolefin fibers which can be used in applications such as inner cover stock for disposable diapers and sanitary napkins. In such applications, the fibers are formed into nonwoven fabrics which have specific property requirements, including soft hand (comfortable touch to the skin), light-weightness and high tensile strength. The fibers can be bonded together to form a nonwoven fabric by several conventional techniques. The needle punch method, for example, interlaces fibers to bond them into a fabric. Fiber binding has also been achieved by depositing a solution of adhesive agent on webs of the fibers, but this requires additional processing and energy to remove the solvent from the adhesive agent. Another approach has been the use of binder fibers having a lower melting point than the primary bulk fibers in the fabric. The binder fibers are heated to fuse to the bulk fibers and produce the nonwoven fabric.
Various attempts have been made in the prior art to employ polyethylene in the manufacture of fibers. Fibers containing polyethylene and polypropylene have been used to manufacture nonwoven fabrics. Polypropylene fibers are known for their high strength and good processability, but suffer from a lack of softness (poor hand). Polyethylene, on the other hand, is known for its good hand, but has poor strength and processability. Blending the polyethylene and polypropylene to form fibers having a good balance of properties has been a long sought goal, i.e. a polyolefin with the hand of polyethylene, but having the strength and processability characteristics of polypropylene. However, problems have been encountered in the manufacture of polyolefin fibers containing both polyethylene and polypropylene. Low density polyethylene (LDPE) and high density polyethylene (HDPE) have been used as bicomponent fiber-forming polymers but are not popular because nonwoven fabrics produced using these polyethylenes have unsatisfactory rigid hand and do not feel soft. Linear low density polyethylene (LLDPE) and polypropylene are generally immiscible and incompatible. Biconstituent fibers containing them generally have a "bicomponent" morphology, i.e. the polyethylene and polypropylene are present in the fibers in co-continuous phases (side-by-side or sheath/core) rather than a dispersion of fibrils of one constituent in a matrix of the other. This has in turn led to various processing problems which are generally addressed by the judicious selection of polyethylene and polypropylene having a specific density and melt index or melt flow ratio.
U.S. Pat. No. 4,874,666 teaches biconstituent fibers produced by melt spinning a blend comprising more than 50 weight percent of a linear low density polyethylene (LLDPE) having a melt index (MI) of 25-100 dg/min and heat of fusion below 25 cal/g, and less than 50 weight percent of crystalline polypropylene having a melt flow rate (MFR) below 20 dg/min. It is stated that these fibers can be produced at relatively high spinning rates. However, it is taught that if the MI of the LLDPE is below 25, fibers cannot be made by high speed spinning, and if the MI of the LLDPE is higher than 100, its viscosity does not match the polypropylene so that a uniform blend cannot be obtained during melt spinning and a serious defect will take place in that the filaments being extruded will frequently break as they emerge from the spinnerette. It is similarly taught that the LLDPE must have the low heat of fusion in order to obtain a uniform blend. Similarly, it is taught that a crystalline polypropylene cannot have an MFR exceeding 20 or uniform blending with the LLDPE cannot be obtained by any of the known commonly employed spinning apparatus, and as a result, great difficulty is involved in spinning the blend at high speed. It is also taught that the LLDPE in the spun fibers is a continuous phase and the polypropylene is a dispersed phase, and that too great a difference in the melt viscosities between the LLDPE and polypropylene results in the dispersed polypropylene particle size being too large for smooth high-speed spinning.
U.S. Pat. No. 4,839,228 discloses a two-part blend of polypropylene with 20 to 45 wt. % LLDPE or alternatively LDPE or HDPE for the production of fibers.
U.S. Pat. No. 4,748,206 discloses a four-part blend of 20 to 70 weight percent crystalline polypropylene, 10 to 50 weight percent amorphous copolymer (EPR), 5 to 50 weight percent ethylene/alphalpha-olefin copolymer, typically ULDPE and 5 to 30 weight percent LLDPE or HDPE to be used for molded articles.
U.S. Pat. No. 4,634,735 discloses a three-part blend of 50 to 97 wt. % isotactic polypropylene, 2 to 49% elastomer (EPR) and 1 to 30 wt. % LLDPE with a density of up to 0.935 for production of molded articles.
JP 9043-043-A discloses a three-part blend of 100 parts by weight polypropylene, 3 to 10 parts by weight LLDPE, and 5 to 15 parts by weight of elastomer, typically EPR for production of film.
U.S. Pat. No. 4,833,195 discloses a three-part blend of an oligomer or degraded polyolefin, typically polypropylene, blended with an olefinic elastomer, typically EPR, and thermoplastic olefin with functional group which is typically LLDPE for the production of films and fabrics.
The latter four references all disclose blends containing elastomer rather than plastomer. As will be discussed below plastomers have significant differences from elastomers. Briefly, the plastomers of this invention have higher crystallinity than elastomers which translates to unexpectedly greater strength and abrasion resistance properties, among others.